The present invention relates to an image display system, in particular, relates to a high quality image display system which is used in an organic EL (Electro Luminescence) display panel.
In recent years, an image display panel using an organic EL (Electro Luminescence) element has been developed. When an organic EL panel which has a large number of organic EL elements is activated by using an active matrix circuit, a pixel of each EL is coupled with an FET (Field Effect Transistor) which is implemented by a TFT (Thin Film Transistor) for controlling supply current to each pixel. Therefore, each pixel is associated with a first TFT which biases an organic EL element to allow current to flow in said organic EL element, and a second TFT transistor for switching said first TFT transistor.
FIG. 9 shows a circuit diagram of a prior active matrix type organic EL display panel. The organic EL display panel comprises first signal lines in X-direction 301-1, 301-2, et al, second signal lines in Y-direction 302-1, 302-2, et al, power supply (Vdd) lines 303-1, 303-2, et al, switching transistors (TFT) 304-1, 304-2, et al, bias transistors (TFT) 305-1, 305-2, et al for controlling bias current in an EL element, organic EL elements 306-1, 306-2, et al, capacitors 307-1, 307-2, et al, a drive circuit in X-direction 308, and a drive circuit in Y-direction 309.
A pixel is selected by an X-direction signal line 301, and a Y-direction signal line 302, and the switching TFT transistor 304 associated with the selected pixel is turned ON so that the capacitor 307 associated with the selected pixel holds image data. Therefore, the bias TFT transistor 305 is turned ON, then, the organic EL element 306 is supplied with bias current associated with image data from a power supply line 303 so that the organic EL element 306 emits light.
When a signal line 301-1 in X-direction receives a signal associated with image data, and a signal line 302-1 receives a scanning signal in Y-direction, a switching TFT transistor 304-1 which is selected by said signal lines 301-1 and 302-1 is turned ON, then, a bias TFT transistor 305-1 is turned ON by a signal associated with image data, so that the selected EL element 306-1 is supplied bias current, and said EL element 306-1 emits light.
Therefore, each pixel of active matrix type EL display panel comprises a thin film type EL element, a bias TFT transistor for controlling light emit of said EL element, a signal hold capacitor coupled with a gate electrode of said bias TFT transistor, and a switching TFT transistor for writing data in said capacitor. The light intensity of each EL element depends upon current flowing in a bias TFT transistor, and said current depends upon voltage charged in a signal hold capacitor. This is described in (A66-in 201 pi Electro luminescent Display T. P. Brody, F. C. Luo, et al., IEEE Trans. Electron Devices, Vol. ED-22, No.9, Sep. 1975, pages 739-749).
The capacitance of a signal hold capacitor must be small so that a switching TFT transistor can charge the capacitor in a short selected time, and simultaneously, must be large so that the voltage across the capacitor does not decrease by the leak current through switching transistors which are not selected thereby for keeping excellent image quality until the next selection of a pixel.
By the way, an active matrix type display panel is requested having the size larger than 4 inches because of visibility when no optical enlarge system is used. A silicon single crystal substrate is impossible to implement that size of panel, because of high cost, since each sheet of single crystal Si substrate can provide very small number of panels under the current producing technology.
Therefore, it is preferable that an active matrix type display panel is comprised of a thin film transistor (TFT) by a non-single crystal Si semiconductor produced on a plane substrate, such as a glass substrate.
As for a non-single crystal semiconductor element on a plane substrate, an amorphous silicon layer has been used because large area of panel is easily obtained. However, a TFT transistor produced in an amorphous silicon layer has the disadvantage that a picture quality is deteriorated, since the threshold voltage of the transistor drifts when current in one direction is flown in a transistor for a long time. Further, as the mobility of a transistor which is produced in an amorphous silicon layer is small, the current with high speed response must be small. Further, as a P-channel is difficult, even a small size of CMOS circuit is impossible in an amorphous silicon layer.
Accordingly, it is preferable to use a poly-crystalline silicon semiconductor layer for an active matrix type organic EL display panel, since relatively large area of panel is possible, with high reliability and high mobility, and CMOS circuit is possible.
A TFT transistor produced in a prior poly-crystalline silicon layer has the disadvantage that trap level density depends upon a number of crystal boundaries in a channel, and that change of the trap level density affects the characteristics of the transistor. Therefore, when the channel length, and/or the channel width of a transistor is close to crystal grain diameter, the variation of a number of grain boundary in a channel is large. This causes the increase of variation of trap level density in a channel, and increase of variation of characteristics of a TFT transistor. The increase of the variation of the characteristics of a TFT transistor deteriorates of picture quality of a display panel.
Another disadvantage of a prior EL display panel concerns that light is obtained through a transparent electrode on a substrate. The reason of that is described in Japanese patent laid open publication 234683/1996, paragraph [0011] that (1) a non-transparent metal such as MgAg alloy which has work function less than 4 eV must be used in an anode electrode of an organic EL element, (2) the patterning of that metal is difficult, and (3) an organic thin film for a light emit layer and/or a transportation layer of a carrier is damaged and layers are separated in a photo-resist process using organic solvent.
Because of the above reasons, the separation of pixels in an organic display panel is carried out by patterning a transparent cathode electrode which is produced before an organic layer is produced, so that light is obtained through a substrate. Further, a non-transparent element, including a TFT transistor, a capacitor, and/or a lead line, prevents light, therefore, those elements are produced outside of an organic EL element.
FIG. 10A shows a pixel of an active matrix type organic EL panel, comprising a switching TFT transistor 304, a bias TFT transistor 305, an organic EL element 306, and a signal hold capacitor 307.
The light intensity emitted by a pixel depends upon area (xcex1) of a pixel. It is assumed that a pixel of a passive type organic EL element 406 has the area (xcex1) as shown in FIG. 10B. In an active matrix type organic EL element, the area for light emission is the area (xcex2) which is the area of an organic EL element 306, and (xcex2) less than (xcex1).
When there is no switching transistor 304, no bias transistor 305, and no capacitor 307, FIG. 10A coincides with FIG. 10B.
It is assumed that the light intensity for each unit area in FIG. 10B is A, and the light intensity for each unit area by an organic EL element 306 in FIG. 10A is B.
When the total intensity of a pixel in FIG. 10A is equal to that in FIG. 10B, the following relations are obtained.
(xcex2)B=(xcex1)A
B=(xcex1/xcex2)A
An active matrix type organic EL element must have higher light intensity for each unit area of an organic EL element 306 if the same light intensity as that of the passive EL element 406 is desired, because of the presence of a switching TFT transistor 304, a bias TFT transistor 305 and a signal hold capacitor 307.
In other words, the components in a pixel decreases the ratio of area of an organic EL element in a pixel, and therefore, an organic EL element must have higher light intensity in order to compensate said decrease.
The increase of light intensity must follow the increase of current in an organic EL element, however, large current in an organic EL element is undesirable because large current shortens the lifetime and decreases operational reliability of an organic EL element.
It is an object of the present invention to provide a new and improved image display system by overcoming the disadvantages and limitations of a prior image display system.
It is an object of the present invention to provide an image display system which provides uniform light intensity on a whole panel with less variation of light intensity.
It is also an object of the present invention to provide an image display system which can omit a capacitor for keeping data to be displayed in a pixel.
The above and other objects are attained by an image display system having a plurality of thin film image display elements for light emission, each of said display elements being cyclically energized by a D.C. power source sequentially, comprising; a bias thin film transistor coupled between an image display element and a D.C. power source for flowing current into said image display element according to image data to be displayed; said transistor having a source, a drain and a gate, being formed on a substrate having an active layer; an active layer of said bias thin film transistor being formed by poly-crystalline silicon; length and width of a gate in said active layer of said bias thin film transistor being at least 10 times as large as average diameter of crystal grain of poly-crystalline silicon which forms said active layer.
Preferably, gate capacitance of said bias thin film transistor is large enough for keeping gate voltage until energization of the transistor in next cycle.